A typical telecommunications network comprises a number of nodes interconnected by communications paths. The nodes incorporate equipment that requires commissioning on installation and periodic servicing either to effect repairs or to install new software. To facilitate this exercise, it is conventional to provide a voice communications channel between nodes to allow service engineers to communicate with each other. This voice channel, which is independent of the network and may thus be used during installation and testing, is generally referred to as an engineering order wire (EOW). Such a facility is of particular importance in synchronous (SDH or SONET) networks where it is necessary for engineers located at various system nodes to ensure synchronisation of those nodes when the system is set up.
Traditionally, when a long distance link comprising end stations and intermediate regenerator stations is installed, the engineering order wire is provided in the form of a "party line" which is private to the associated long distance link. Telephone terminals are provided at the end stations and at the regenerator station, and an engineer lifting the handset of any one of those telephones Is able to hear any speech on the line. To support the operation, means are provided for attracting the attention of personnel at the different sites along the line, e.g. by use of a simple code of one ring for site one, two rings for site two and so on.
The introduction of synchronous technology has greatly increased both the distances and the transmission rates that can be achieved by carrying the traffic over optical fibre links. Rather than go to the expense of providing a physically separate engineering order wire connection, it has been proposed to provide this facility in the form of overhead channels. Typically, two 64 kbit/s channels are allocated for this purpose. These channels are extracted/inserted at all sites where an order wire facility is to be provided. This arrangement however suffers from two disadvantages. Firstly, the engineering order wire traffic is carried on the main fibre transmission path. If that path is cut, then the engineering order wire communication is also lost at a time when it is most needed. Secondly, network topologies have evolved to such an extent that engineering communication along a single unbranched path is no longer adequate. For example, present day networks can now comprise meshes or hierarchies of ring systems. These complex topologies permit the formation of loops when an engineering order wire is connected as a party line. This can lead to positive feedback or "howl" preventing effective communication.